CS420: Operating Systems. Kernel I/O Subsystem

Transcription

1 Kernel I/O Subsystem James Moscola Department of Physical Sciences York College of Pennsylvania Based on Operating System Concepts, 9th Edition by Silberschatz, Galvin, Gagne

2 A Kernel I/O Structure 2

3 Kernel I/O Subsystem The kernel provides many services related to I/O - Scheduling - Buffering - Caching - Spooling - Device reservation - Error handling 3

4 Kernel I/O Subsystem Scheduling - Some I/O request ordering via per-device queue Attempt to use devices optimally while still providing priority - Some implement Quality of Service (i.e. IPQOS)! Buffering - store data in memory while transferring between devices - Helps cope with device speed mismatch - Helps cope with device transfer size mismatch - To maintain copy semantics Data is first copied from user application memory into kernel memory Data from kernel memory is then written to device Prevents application from changing contents of a buffer before it is done being written 4

5 Kernel I/O Subsystem Caching - possible to reuse buffers in main memory for caching - Cache files that are read/written frequently - Increases performance! Spooling - a buffer that holds output for a device that cannot accept interleaved data streams - If device can serve only one request at a time - Allows a user to see individual data streams and remove streams if desired - i.e., Printing! Device reservation - provides exclusive access to a device - System calls for allocation and de-allocation - Watch out for deadlock 5

6 Error Handling OS can recover from disk read, device unavailable, transient write failures - Retry a read or write, for example - Some systems more advanced Solaris FMA, AIX Track error frequencies, stop using device with increasing frequency of retryable errors Most operating systems return an error number or code when I/O request fails - errno in UNIX/Linux environments System error logs hold problem reports 6

7 I/O Protection User process may accidentally or purposefully attempt to disrupt normal operation via illegal I/O instructions - All I/O instructions defined to be privileged (i.e. kernel must perform I/O) - I/O must be performed via system calls Memory-mapped and I/O port memory locations must be protected too 7

8 Kernel Data Structures Kernel keeps state info for I/O components, including open file tables, network connections, etc.! Many complex data structures to track buffers, memory allocation, dirty blocks! Some operating systems use object-oriented methods and message passing to implement I/O - Windows uses message passing Message with I/O information passed from user mode into kernel Message modified as it flows through to device driver and back to process 8

9 UNIX I/O Kernel Structure 9

10 I/O Requests to Hardware Operations Consider reading a file from disk for a process: - Determine device holding file - Translate name to device representation - Physically read data from disk into buffer - Make data available to requesting process - Return control to process 10

11 Life Cycle of An I/O Request In buffer cache? 11

12 Performance I/O a major factor in system performance: - Demands CPU to execute device driver, kernel I/O code - Context switches due to interrupts - Data copying - Network traffic especially stressful 12

13 Intercomputer Communications 13

14 Methods to Improve System Performance Reduce number of context switches Reduce data copying when passing data between device and application Reduce frequency of interrupts by using large transfers, smart controllers, polling Use DMA controllers to offload simple data copying from the CPU Use smarter hardware devices that can do more of the work Move user-mode processes / daemons to kernel threads 14

15 Device-Functionality Progression 15